1. Field of the Invention
The present invention relates generally to the synthesis of metal powders and films, and more specifically, to the continuous synthesis of nanostructured metal powders and coatings using a microwave waveguide, cavity or beam system.
2. Description of the Related Art
Metallic powders have been prepared by physical vapor deposition, by mechanical blending and mixing, and by chemical routes. Vapor methods are not cost effective and make only small amounts of material. The mechanical blending route often introduces impurities into the final product. Fluidized beds have also been used to coat powders with metals, however, as in vapor methods, the initial equipment is expensive, and it is difficult to evenly coat powders and to handle powders of different sizes.
Metallic coatings have been prepared using electroplating and electroless plating. Electroless plating requires that the substrate be pretreated before plating and the substrate must also be an insulator. Using the polyol method, there does not need to be any chemical pretreatment of the surface and the substrates may be either conductive or insulators.
Nanostructured powders and films (with particle diameters of about 1-100 nm) have many potential electronic, magnetic, and structural applications such as catalysis, electromagnetic shielding, ferrofluids, magnetic recording, sensors, biomedical, electronics, and advanced-engineered materials.
Among the various preparative techniques, chemical routes offer the advantages of molecular or atomic level control and efficient scale-up for processing and production. Others in the art have prepared micron and submicron-size metallic powders of Co, Cu, Ni, Pb, and Ag using the polyol method. These particles were composed of single elements. Depending on the type of metallic precursors used in the reaction, additional reducing and nucleating agents were often used. The presence of the additional nucleating and reducing agents during the reaction may result in undesirable and trapped impurities, particularly non-metallic impurities.
These prior procedures have been unable to obtain nanostructured powders having a mean size of 1-25 nm diameter. These prior procedures have not been useful in producing nanostructured powders of metal composites or alloys or metal films.
U.S. patent application Ser. No. 10/113,651 to Kurihara et al. discloses the use of millimeter wave radiation to heat a polyol reaction mixture in a batch process. This process allows for the production of nanostructured metal powders and films via the polyol process.
Grisaru et al., xe2x80x9cPreparation of Cd1-xZnxSe Using Microwave-Assisted Polyol Synthesis,xe2x80x9d Inorg. Chem., 40, 4814-4815 (2001), discloses the use of low power microwave radiation to make small hatches of metal nanoparticles. The yield of this process is less than single gram quantities per batch.
There remains a need for a process for making large quantities of nanostructured metal particles and films using the polyol process
The invention comprises a method of forming a nanocrystalline metal, comprising the steps of: providing a reaction mixture comprising a metal precursor and an alcohol solvent; continuously flowing the reaction mixture through a reactor; applying microwave or millimeter-wave energy to the reaction mixture; wherein the microwave or millimeter-wave energy is localized to the vicinity of the reaction mixture; and heating the reaction mixture with the microwave or millimeter-wave energy so that the alcohol solvent reduces the metal precursor to a metal; wherein the heating occurs in the reactor.